Nuclear power plants are designed to withstand the ground motion caused by the most severe earthquake that is likely to be experienced. Seismic analysis must show that the reactor can be safely shutdown in the event of such an earthquake. The components and structures vital to safe shutdown are referred to as Safety Class I structures.
When rock is located at great depths from the natural ground surface (in excess of 30 meters or about 100 feet), it is very costly and time consuming to provide Safety Class I foundations utilizing conventional building mat foundations. The seismic accelerations that emanate from the rock boundaries are significantly amplified through the soil, resulting in very high seismic forces on the structures, systems and components of a nuclear power plant. The liquefaction potential under seismic excitation severely impacts the underground foundations in conventional designs.
Thus, there is a need for a new construction scheme that overcomes weak soil foundation material overlying deep rock on sites selected for nuclear power plants. This construction scheme must minimize the effect on power plant structures, systems and components of seismic events. Also the scheme should be able to accommodate different types of plants.